For example, the fan coupling (fan clutch) consists of a body, which has a disc at the end of a shaft of a pump and a wheel at the end of a shaft of a fan such that the disc and the wheel are faced each other. The disc rotates with the rotation of the pump, while immersing in a viscous operating oil which is filled in the body, whereby the torque is transmitted to the wheel through the operating oil and as the result, the fan rotates.
As the operating oil for fan coupling, organopolysiloxane such as dimethylpolysiloxane and methylphenylpolysitoxane, generally having high viscosity index, is used. The organopolysiloxane is easily subjected to thermal degradation or gelation when used at high temperature because of its poor thermal stability, whereby its viscosity increases and its torque transmission ability cannot be maintained stably.
For improving the thermal stability of organopolysiloxane, it has been proposed to add an antioxidant such as iron octoate, aromatic amine derivatives and ferrocene derivatives in organopolysiloxane. When the composition comprising organopolysiloxane and the antioxidant was practically used in the fan coupling, however, the increase in viscosity was observed.